You're being bitten by precedence. Ternary ?: has a higher precedence than =, so what perl parses is:

Y:\tmp>perl -MO=Deparse,-p 470538.pl
(my(%hash) = ('A', 'on', 'B', 'on', 'C', 'off', 'D', 'off', 'E', 'off'
+));
foreach $_ (keys(%hash)) {
    my($result);
    (((exists($hash{$_}) && ($hash{$_} eq 'on')) ? ($result = 'OK') : 
+$result) = 'KO');
    print($result, "\n");
}
470538.pl syntax OK
[download]

You're effectively doing ($result='OK')='KO' in the case of a match, hence your output. The solution is of course to use parentheses:

   (exists $hash{$_} and $hash{$_} eq 'on') 
      ? ($result = 'OK') 
      : ($result = 'KO');
[download]

Nice gotcha you found there :)